XPS and ion scattering studies on compound formation and interfacial mixing in TiN/Ti nanolayers on plasma nitrided tool steel

The adhesion improvement produced by bright plasma nitriding hardening of tool steel prior to TiN deposition was investigated by angle-resolved X-ray photoelectron spectroscopy and atomic profiling with sub-nanometric depth resolution. Steel hardening is considered to be a consequence of homogeneously distributed, very fine iron and chromium nitride precipitates existing in the N-diffused zone formed below the surface by bright plasma nitriding, a picture confirmed here by the chemical analysis of the near-surface regions of the steel substrate. Besides hardening, analysis of the O1s, N1s, Ti2p, Fe2p, and Cr2p photoelectron groups showed that the finely dispersed chromium and iron nitrides produced by plasma nitriding also replace in part the oxides and hydroxides in the near-surface regions. Ti profiling with medium energy ion scattering and N profiling with narrow resonant nuclear reaction pointed out the interdiffusion of at least these two species near the interfaces. Thus, the enhanced adhesion and improved performance observed in duplex treated tool steel are discussed in light of the present findings: i) stronger chemical bonding at the coating/substrate interface promoted by bright plasma nitriding of the steel substrate prior to TiN deposition and ii) enhanced interdiffusion around the coating/substrate interface owing to the removal of the oxides which constitute diffusion barriers, producing an enlarged contact area and more gradual coating/substrate interfaces.